We've been using Formatt Hitech ND filters for a long while now, but until recently we'd been using the resin filters. Given the lack of horizontal/vertical axes in our shots, we don't use graduated filters - just solid ones.
Recently we took delivery of a new set of Firecrest ND 165mm filters - picking up 3, 6, 9, 10, 13 & 16 stop filters. These use a metallic filament sandwiched between layers of Schott Superwite glass - and provide both great optical clarity and enhanced scratch-resistance as a result. (formatt-hitech.com)
Next weekend will see our first big test of the new filters, followed by a series of trips to Tasmania, Queensland and Western Australia starting in March. Early results tell me that although we've been really happy with the resin filters, the glass is even better. And as we're now moving to medium format cameras for the landscape shots, this ought to be a big plus.
Now that we can take really long exposures without noise (thanks to a PhaseOne iQ260 back), the higher-stop filters should really come into their own. That said, all of the Hitech filters we've used have been extremely neutral - and if there is any colour-caste at all, it is consistent and very easily adjusted for.
Stay tuned for further updates as we really put the filters through their paces..